Mercury switch



Feb. 25, 1941. c. H. LARsoN MERCURY SWITCH Filed Jan. 12, 1939 Patented Penas, 1941 Carl H. Larson, Elkhart, Ind., assignor to The;

e Company, a corporation of Illinois Application January 12, 1939, Serial No. 250,521

23 Claims.

'Ihis invention relates primarily ,to mercury switches of the type in which a magnetically controlled plunger is mounted within a hermetically sealed envelope. containing mercury and spaced electrodes. to displace more or less mercury for changing the condition of the electrical: circuit through the electrodes. The invention, however, is not limited in its use' to mercury switches of this type, but may also be used in conjunction with tilting switches and even other types of mercury switches.

More specifically, the invention pertains to the construction of the intermittently immersed conv tact of a mercury switch, and among its principal objects are the following: To provide an elecifi trode construction that will stand up almost indefinitely under all conditions of wear and usage; to produce a switch that will consistently give sharply defined makes and breaks; to form the electrode in such a manner that the operating characteristics of the switch with its associated relay coil may be controlled by slight variations in the electrode construction; to provide an electrode construction that will give uniform results, particularly in time delay relays; to increase the current carrying capacity for a switch of given size; and to provide an electrode construction that can be fabricated at low cost.

Further and other objects and advantages will become apparent as thedisclosure proceeds and the description is read in conjunction with the accompanying drawing, in which Fig. 1 is a sectional view of a fast acting plunger type mercury switch with a mercury 5 to mercury contact, the central electrode consisting of a ceramic cup that is bonded to the glass sleeve surrounding the central electrode;

Fig. 2 is a fragmentary view showing a slightly modified form of central electrode; v

Fig. 3 shows still another modification of the central electrode;

Fig. 4 is a fragmentary view showing a metal to mercury contact which incorporates some of the vfeatures of this invention; 5 Fig. 5 is a sectional view of a time delay relay embodying the present invention;

Fig. 6 is an enlarged view of the electrode construction shown in Fig. 5;

tion; and v Fig. 8 is a top plan view of the electrode construction shown in Fig. 7.

This application is a continuation in part of my earlier application, Ber..No. 82,855, filed June .i 1, 1936.

In a mercury switch, it is customary to have one electrode that is constantly in contact with the mercury fill andV another electrode which is intermittently placed in contact with the mercury ll by some means for manipulating the li. I'he relative position. of the lill with respect to the electrodes may be effected by tilting the switch envelope,l or by displacing mercury by means of a magnetically controlled displacer.

When the mercury switch is being used for controlling circuits having a relatively heayy load, there is a tendency for the intermittently immersed electrode to become damaged by the intense heat and destructive effect of the arc formed at the instant when the circuit is opened or closed, and particularly when the circuit is being opened under an inductive load. Various attempts have been made to protect the electrode by' the use of refractory materials, but none of these have the simplicity and effectiveness -of the present invention. Another means for protecting the intermittently immersed electrode is by bending the bared end of the electrode over the insulating sleeve, as shown in my prior Patent No. 1,967,247.

The nature of the invention and itsutility will become apparent by referring to Fig. 1 of the drawing in which one form of the invention is shown. The switch envelope 'I0 is made of glass and contains a quantity of mercury II. A

. pair of electrodes I2 and I3 are sealed through the bottom oithe electrode, the latter electrode being enclosed within a glass sleeve I4 for a portion of its height. The sleeve Il has a 4bulge l which acts as a seat for a spring I6, the upper end of which is spread out as indicated .at I1, to yieldingly restrain the displacer, generally designated I8, from striking .the bottom of the switch envelope.

The displacer I8 comprises an inner sleeve I 9 of glass, ceramic, or other light-weight nonmagnetic material, and an outer sleeve of magnetic material. 'I'he inner sleeve I 9 has its lower end enlarged to form a shoulder 2I on which the outer sleeve 20 ls mounted. Guide washers are carried by the displacer for guiding its movement within the switch envelope, the top washer being held in placeby a coil spring 28.

A ceramic cup 21 is mounted on the upper f exposed end of the electrode I3. 4I'he cup contains a pool of mercury .28 which is in contact Fig. 7 lshows a preferred electrode construcwith the bared end of the electrode I8. In practice, the electrode assembly is made by rst taking a suitable length of electrode wire I3, preferably of molybdenum, then telescoping the -glass sleeve I4 over the wire, heating the two erably the materials for the glass sleeve Il and the ceramic cup are chosen in accordance with the disclosure of my prior application, Ser. No. 82,855, filed June 1, 1936 (which application is hereby specifically made a part of the present disclosure), so that they may be fused together.

When this method of bonding is used, sufficient heat` is applied to cause the glass to flow into the interstices of the ceramic material and the result is a bond that is exceptionally strong. A bond of this kind also has the advantage that there is no opportunity for a short circuit to occur in the switch at the bond 23 since that bond is substantially the equivalent of a unitary structure.

A relay coil 29 is associated with the switch, and when the coil is energized, the displacer I3 is drawn downwardly, causing the mercury level to rise above the lower edge of the cup 21 and make contact with the mercury pool 23, thus closing the circuit between the electrodes I2 and I3 through the body of mercury H. When the coil is de-energized, the displacer rises and the mercury level falls, thus breaking the circuit.

The cup or cap 21 is an integrally formed piece of ceramic material, such as Alundum (grade RA84, RA360 or RA98), which, I am informed, is substantially pure A1203. It is a product of the Norton Company, Worcester, Massachusetts. Thel cap 21 is preformed into its desired shape, preferably by being molded, and the passageway 22 is of a size which permits the cap to tightly telescope the bared end of the electrode I3.

It has been found that when the cup 21 is provided with a flat horizontal rim as distinguished from the sloping rim 35 shown in Fig. 1, there is a tendency for the mercury 23 within the cup to cling to the main body of mercury I I at the instant that the circuit is being broken, with the result that the break is not clean, or in other words, it is characterized by a chatten Presumably this is caused by the fact that the mercury 23 in the cup 21 at the instant when the mercury fill II has been lowered by the displacer 4I3 to a point where it is about to sepa.

rate from the mercury pool 2l, is connected to the main body of the fill II on all sides, so that when the mercury fill II is lowered to the extent that a break is caused. the break does not occur instantaneously on all sides of the cup, but instead occurs on one side only, causing the mercury 2B in the cup to snap back to the other side of the cup and prolong the time required for the breaking of the circuit. This snapback" action is not smooth, and it usually produces a. series of breaks on the side of the cup toward which the movement of mercury is directed, and this characteristic vjagged break or chatter is clearly indicated on high speed photographs or similar equipment.

The same characteristic action takes place in the electrode structure shown in Fig. 2 in which the rim of the cup has a double slope providing low points 35 and 31 along the rim. Obviously the break will first take place at one or the other of these two low points, and the snap back action of the mercury in the cup will produce a more or less jagged break of the circuit on the other side of the cup, but the action is not nearly as `pronounced as with a cup having a horizontal flat rim.`

These unfavorable switch characteristics are avoided by the use of a single sloped cup such as shown in Fig. 1, in which the break between the two bodies of mercury is confined to a relatively small portion of the rim of the cup 21. The cup is truncated, as shown at 35, providing a low portion 39 which invariably is the edge over which the break occurs. Any snap back action of the mercury in the cup 21 is directed against the opposed wall Il of the cup and this wall is sufficiently high so that the mercury 23 cannot be thrown over the wall by the rush of mercury to the left (Fig. 1) at the instant when the connection between the body of mercury in the cup 21 and the mercury fill II in the switch envelope I0 is broken.

The same result may be accomplished by using a cup 4I (Fig. 3) having a horizontal rim 42 provided with a cut-outportion 43. Obviously when the level of the mercury body II falls away from the rim 42 of the cup, the.break between the body of mercury in the cup and the main mercury fill will occur over the edge 4l and the opposite wall being considerably higher will prevent the mercury from sluiling over to cause chattering on the breaking of the circuit.

Electrodes of the mercury cup type, when constructed according to the teachings of this invention to localize the break lat a depressed portion of the rim, also have the advantage that they enable the switch as a whole to operate on relatively small differences between pickup and drop-away values for the associated relay coil, particularly when considering the usual relatively high differences between these values in relays using mercury to mercury contacts. For example, a switch using an electrode provided with a horizontal fiat rim (but without a depressed portion as in Fig. 3) would have a relatively high diflerential between pickup and drop,

away values.

By varying the angle of the sloping rim 35, the span between pickup and drop away values may be controlled.` When the angle is very steep, the span is relatively low, and when the' angle is small (i. e.) approaching the horizontal, the span is relatively large.

The same control of the span may be effected in the electrode construction shown in Fig. 3 by varying the width of the cut-out portion 43. When .a relatively large section is cut away, the span between pickup and drop-away values is correspondingly large.

A time delay switch is shown in Fig. 5 which illustrates other features of the invention. The switch comprises an envelope containing a mercury fll 5I. Conductors 52 and 53 are sealed through the base of the envelope and are covered for a portion of their lengths by insulating sleeves 54 and 55, respectively. The latter is cut away, as indicated at 55, so that the conductor 53 is in constant contact with the mercury fill 5I. Fused to the upper ends of the sleeves 54 and 55 is a glass thimble 51 provided at its upper portion with a porous wall 5B preferably of ceramic material such as Alundum, and having a relatively large opening 59 for admitting mercury to the interior ot the thimble. The conductor 52 with its insulating sleeve 5l extends upwardly into the thimble 51, the sleeve being enlarged at its upper end as indicated at 60 to provide a seat for the ceramic cup 5I which surrounds the exposed end 32 of the conductor 52. The cup 5I is preferably a preformed ceramic piece cut from cylindrical tubing and the cup is suitably bonded to the enlarged head 50 of the sleeve 54.

The displacer 53 is similar to the displacer I8 in Fig. 1 and when it is pulled down bythe associated relay coil, a given quantity of mercury is displaced which normally raises the outside mercury level to at least the upper end 64 of the thimble 51, although the admission of mercury to the interior of the thimble 51 is restrained by the slow escapement of gas trapped within the thimble through the pores of the ceramic plug 58.

When the displacer 63 is released by the coil, it immediately rises to the position shown in Fig. 5, and the outside mercury level falls to a point well below the upper margin 65 of the opening 59, with'the result that the mercury in the thimble 51 immediately falls from the thimble. breaking the circuit between the electrodes 52 and 53.

In normal operation of the switch, the displacer 53 is held down by the relay coil for a vsuiilcient time to vpermit the mercury entering the thimble 51 to reach the top of the thimble so that al1 gas is expelled from the thimble. It sometimes happens, however, that the relay coll is de-energized before the mercury level has had a chance to completely fill the thimble, .and in such a case, the amount of mercury which rushes by the cup 6I when the circuit is being broken is substantially less than the head of mercury which passes the cup when the displacer is released following a complete filling of the thimble 51. The result is that when the thimble is emptied subsequent to '.a complete illling of the thimble with mercury, the amount. of mercury remaining in the cup 5l may be represented by the dotted line 66 in Fig. 6 whereas the amount of mercury remaining in the cup when the thimble is emptied subsequent to a partial filling of the thimble is represented by the full line 51 in Fig. 6. Obviously, this difference in the amount of mercury left in the cup alects the time delay characteristics of the switch and produces a certain amount of non-uniformity even though it has the advantages of the electrode structure shown in Fig. 1 and the additional advantage arising from the ease with which it can be fabricated from cylindrical tubing stock of ceramic material.

I have found that I can eleminate this undesirable variation in timing by using the'electrode structure shown in Figs. 7 and 8. 'I'his structure differs from that shown in Fig. 5 in two respects: First, in that the conductor 52 extends above the maximum mercury level contained within the cup 68 and second, in that the bared end 69 of the conductor 52 is eccentrically positioned within the cup 68.

Both of these features have advantages which make them useful in fast acting switches as well as slow acting switches, and for this reason, the electrode structure of Figs. 7 and 8 constitutes a preferred form of the invention. By having the bared end 69 of the electrode 52 extending above the maximum mercury level within the cup -68 a suilicient amount, the contact resistance between the mercury and the exposed end 69 of the electrode 52 may be made substantially equal to or less than the resistance of the conductor 52 itself. In other` words, by extending the bared end 69 of the conductor 52 a sucient distance, it is possible to have the area ofthe'contact between the bared end 69 and the mercury in the envelope of such extent that the resultant contact resistance is substantially equal toA or less than the resistance of the electrode 52, with the result that there is no tendency for the mercury in contact with the electrode 6 9 to become hot, or at least any hotter than the sleeve 5l surrounding the conductor 52. This feature of the electrode construction makes it particularly desirable for use in all types of mercury switches.

The extension of the conductor 52 above the maximum mercury level in the cup 6l has another distinct advantage, particularly when the' electrode is used in a time delay relay such as the one shown in Fig. 5. It has been found that when the bared. end 52 (Fig. 5) of the electrode 52- projects above the mercury in lthe cup 8l, the difference between the mercury level $8 and 61 in Fig. 6 resulting from abnormal operating conditions is greatly reduced. Presumably this phenomenon is due to the change in surface tension occasioned by the projection of the electrode 62 through the mercury illl in the cup 6I.

When the bared end of the electrode 52 ,is placed eccentrically within the surrounding cup, as for example in Figs. 7 and 8, the cup can have a smaller inside diameter without affecting the .ability of the cup to fill after it has been emptied, as, for example, after the switch has been turned upside down. This means that the overall diameter of a switch having a given current carrying capacity can be held to a minimum. 'By way of example, the cup 58 in Fig. 7 will fill consistently .and easily when the diameterof the electrode 69 is 60 mils, and the cup has an inside diameter of of an inch. A cup of the same size will not fill satisfactorily when the electrode is centrally positioned as shown in Figs. 5 and 6. l

The eccentric positioning of the electrode 65 within Ithe cup 68 (Fig. 7) and the resultant ability to reduce the inside diameter of the cu-p 68 has the additional advantage that the quantity of mercury remaining in the cup G8 is always the same, even though the relay is operated under the conditions which produce the abnormal high level 81 shown in Fig. 6, so'th'at the accuracy of a time delay relay using the electrode strucized by the ease with which the electrode structure can be fabricated.

I claim:

1. In a mercury switch, a switch envelope, a quantity of mercury in the envelope, spaced electrodes in the envelope, one of which projects upwardly .through the mercury and is covered for a portion of its length by a sleeve lof insulating material leaving a bared end, means for manipuilating the mercury fill to bridge and unbridge the electrodes with mercury, and an integrally formed cap of refractory material surmounting the insulating sleeve and bonded to it, said cap including a stem portion which tightly telescopes the bared end of the electrode.

2. In a mercury switch, a switch envelope, a quantity of mercury in the envelope, spaced electrodes in the envelope, yone of which projects upwardly through the mercury and is covered for a the insulating sleeve and bonded to it, said cap including a stem portion which tightly telescopes the bared end of the electrode, and a cup portion adapted to contain a quantity oi mercury.

3. In a mercury switch, a switch envelope, a

quantity of mercu-ry in the envelope, spaced electrodes in the envelope, one of which projects upwardly through the mercury and is covered for a portion oi' its length by a sleeve of insulating material leaving a bared end, means for manipulating the mercury lill to bridge and unbridge the electrodes with mercury, an integrally formed cap o! refractory material surmounting the insulating sleeve and bonded to it, said cap in cluding a stem portion which tightly telescopes the bared end oi' the electrode, and a cup portion adapted to contain a quantity of mercury, the cup portion having a part oi its rim lower than the remainder so that -the bridging and unbridging of the electrodes by mercury will take place across said depressed part o! the cup.

4. In a mercury switch, a switch envelope, a mercury lill, spaced conductors in the envelope including an electrode projecting upwardly through the fill and covered for a Aportion of its length by a glass sleeve leaving the end of the electrode exposed, a cup mounted on the sleeve, encircling the exposed end of the electrode and adapted to contain mercury, said cup being of a material more highly refractory than glass, and a magnetically responsive plunger for shifting 'the level of the mercury ll.

5. In a mercury switch, a switch envelope, a mercury illl, spaced electrodes in the envelope, one oi' which projects upwardly through the mercury lill and is covered for a portion of its length by aninsulating sleeveleaving a bared end, a cup sur-mounting the sleeve and bonded to it providing a receptacle for mercury in constant contact with said bared end of the electrode, means for moving the mercury lill to and away from said electrode, said cup having a portion of its 'rim lower than the remaining portion whereby the electrical contact between the body of mercury in the cup and the body of mercury in the envelope is broken across said lower portion of the rim.

6. In a. mercury switch, a switch envelope, a mercury illl, spaced electrodes in the envelope, one of which projects upwardly through the mercury ll and is covered for a portion of its length by an insulating sleeve leaving a bared end, a cup surmounting the sleeve and bonded to it providing a receptacle for mercury in constant contact with said bared end of the electrode, means for moving the mercury fill to and away from said electrode, the rim of said cup having a depressed portion whereby the electrical contact between the body of mercury in the cup and the body of mercury in the envelope is broken across said depressed portion of the rim.

7. In a mercury switch, a switch envelope, a mercury iill, spaced conductors in the envelope including an electrode projecting upwardly through the fill and covered for a portion of its length by an insulating sleeve leaving rthe end of the electrode exposed, a truncated cup surmounting the sleeve and encircling the exposed end o! the electrode and containing mercury. and means for shitting the mercury level in 'the envelope toward and away from the exposed end of the electrode.

8. In a mercury switch, a switch envelope, a quantity of mercury within the envelope, spaced formed cap of refractory material surmountingA electrodes in the envelope, one of which projects upwardly through the mercury ll and is surrounded for at least a portion of its length by a sleeve of insulating material closed below its upper end to form a cup adapted to retain mercury in constant contact with said one electrode, at least a portion oi said sleeve comprising a section of tubing of substantially uniform cross section formed of a ceramic material having highly refractory and arc resistant properties and the upper end of said cup being above the low mercury level, and magnetically responsive means for shifting the mercury iill to bridge and unbridge the electrodes with mercury.

9. In a mercury switch, a switch envelope, a mercury till, spaced conductors inthe envelope including an electrode projecting upwardly through the fill and covered for a portion of its length by an insulating sleeve leaving the end or the electrode exposed, a mercury lled cup surmounting the sleeve and encircling the exposed end oi the electrode, means for shifting the mercury level in the envelope tow-ard and away from the exposed end of the electrode, said electrode being of suiicient height so that its exposed end projects above the mercury in the cup when the electrical circuit through the conductors is open.

10. In a mercury switch, a switch envelope, a mercury fill, spaced electrodes in the envelope, one of which projects upwardly through the mercury fill and is covered for a portion of its length by an insulating sleeve leaving a bared end, a cup surmounting the sleeve and bonded to it providing a receptacle for mercury in constant contact with said bared end of the electrode, means for moving the mercury lill .to and away from said electrode, the bared portion o1' said electrode being sufilcient in extent so that the contact resistance between the mercury in the envelope and the electrode when the switch is closed. is substantially equal to or less than the cross sectional resistance of the electrode itself.

11. In a mercury switch, a switch envelope, a mercury fill, spaced conductors in the envelope including an electrode projecting upwardly through the fill and covered for a portion of its length by an insulating sleeve leaving the end of the electrode exposed, a mercury lled cup surmounting the sleeve and encircling the exposed end of the electrode, means for shifting the mercury level in the envelope toward and away from the exposed end of the electrode, said electrode being extended above the vlowest point of the cup rim whereby a sharp clean-cut break of the electrical circuit between lthe conductors is effected when the mercury level in the switch envelope is dropped.

12. In a mercury switch, a switch envelope, a mercury ll, spaced conductors in the envelope including an electrode projecting upwardly through the illl and covered for a portion of its length by an insulating sleeve leaving the end of rthe electrode exposed, a cup mounted on the sleeve and encircling the exposed end of the electrode and containing mercury, means for shifting the mercury level in the envelope toward and away from the exposed end of the electrode, said electrode being eccentrically positioned within the cup.

' 13. In a mercury switch, a switch envelope, a mercury tlll, spaced conductors in the envelope including an electrode .projecting upwardly through the iill and covered for `a portion of its length by an insulating sleeve leaving the end o! the electrode exposed, a mercury iilled cup surmounting the sleeve and encircling the exposed end of the electrode, means for shifting .the mercury level in xthe envelope toward and away from the expo'sed end of the electrode, said cup having a portion of its rim depressed whereby the electrical circuit between the conductors is normally broken across the depressed vportion of the rim, and said exposed end of the electrode being eccentrically positioned within the cup on the opposite side of said depressed portion of the rim.

14. In a mercury switch, a switch envelope, a mercury lill, spaced conductors in the envelope including an electrode projecting upwardly through the iill and covered for a portion of its length by an insulating sleeve leaving the end of the electrode exposed, said sleevebeing provided with an enlargement, and a tube of ceramic material .bonded to the enlarged portion of the sleeve and forming a cup around the bared end of said electrode;

15. In a mercury switch, a switch envelope, a mercury fill, spaced conductors in the envelope including an electrode projecting upwardly through the lill and covered for a portion of its length by an insulating sleeve leaving the end of the electrode exposed, a mercury lled cup surmountingthe sleeve and encircling the exposed end of the electrode, means for shifting .the mercury level in the envelope toward and away from the exposed end of .the electrode, means for restraining the movement of mercury away from said cup, the bared end of the electrode within the cup extending above the lowest point of the cup rim.

16. In a mercury switch, a switch envelope, a mercury iill, spaced conductors in the envelope including an electrode projecting upwardly through the ll and covered for a portion of its length by anA insulating sleeve leaving the end of the electrode exposed, a mercury filled cup surmounting the sleeve and encircling the exposed end of the electrode, means for shifting the mercury level in the envelope toward and away from the exposed end of the electrode, means for restraining the movement of mercury away from said cup, the bared end of the electrode within the cup extending above the lowest point of the cup rim and being eccentrically positioned with respect to the cup.

17. In a mercury switch, a switch envelope, a quantity of mercury in the envelope, spaced elecltrodes in the envelope, one of which projects upwardiy through the mercury and is covered for a poni-.ion of its length by a sleeve of insulating material leaving a :bared end, means for manipulating the mercury fill to bridge and unbridge the electrodes .with mercury, and an integrally formed cap of refractory material mounted on 'the insulating sleeve and bonded to it, said cap being in the form of tubing and being telescoped over at least a part of the bared end of .the electrode.

18. In a mercury switch, a switch envelope, a mercury fill, contact means including an electrode projecting into the envelope, an insulating sleeve Itelescoped over said electrode, at least the upper portion of said sleeve being formed of highly refractory, arc resistant, ceramic material, and being adapted to retain a quantity of mercury in continuous contact with said electrode and having a portion of its rim lower than the remaining portion, the lowest portion of said rim being above the low level of the mercury, and a magnetically responsive displacer within placer for shifting the mercury level in the envelope toward and away from the exposed end of the electrode-said insulating means having a portion of its rim depressed, whereby the electrical circuit between the conductors is normally broken across the depressed portion of the rim.

20. In a mercury switch, aswitch envelope, a mercury lill, spaced conductors in the envelope including an electrode projecting upwardly through the lill and covered for a portion of its length by an insulating sleeve, leaving the end of the electrode exposed, a mercury iilled cup en` circling the exposed end of the electrode, means for shifting the mercury level in the envelope toward and away from the exposed end of the electrode, said cup having a portion of its rim depressed, whereby electrical circuit between the conductors is normally broken across the depressed portion of the rim.

21. In a mercury switch, a switch envelope, a mercury iill, spaced electrodes in the Aenvelope including an electrode projecting upwardly through the iill and covered for a portion of its length by an insulating sleeve leaving the end of the electrode exposed, a tubular member mounted on .the sleeve and encircling the exposed end of the electrode, and means for shifting the mercury -level in the envelope toward and away from the exposed end of the electrode, said electrode being eccenltrically positioned within the tubular member.

22. In a mercury switch, a switch envelope, a mercury fill, spaced electrodes in the envelope, one of which projects upwardly through the mercury fill, and is covered fora portion of its length by an insulating sleeve leaving a bared end, a cup surmounting the sleeve providing a receptacle for mercury in constant contact with said bared end of the electrode, means for moving the mercury fdl to andaway from said electrode, said cup having at least two portions of its rim lower than the remaining portions, whereby electrical contact between the body of the mercury in the cup and the body of the mercury in the envelope is broken acro said lower portions.

23. In a mercury switch, a switch envelope, a mercury iill, spaced conductors in fthe envelope one of which projects upwardly through the mercury fill and is covered for a portion of its length by an insulating sleeve, leaving a bared end, a cup surmoun'ting the sleeve providing-a receptacle for mercury in constant contact with said bared end 'of 'the electrode, means for moving CARL'H. LARsoN,

CERTIFICATE oF CORRECTION. Patent No. 2,252,871. February 25, 19m.

CARL H. LARsox.

It is hereby Certified that error appears in the printed specification of the above numbered patent requiring correction lsfollows: Page 5, second Column, line 55, claim 21, for the word "electrodes" read --conductors;

line 59, claim 25, for "conductors" read -electrodes; and that the said Letters Patent should be read with this correction therein that the same may conform to the record of the case in the Patent Office.

signed and sealed this 25th day of March, A. D. 191;,1.

l Henry Van Arsdale, (Seal) Acting Commissioner of Patents.

CERTIFICATE oF CORRECTION. A Patent No. 2,252,871 rnbrulry 25, 19in.

CARI. H. mason.

l: is hereby Certified that error appears in the printed specification of the above numbered patent requiring correctionls. follows: Page 5., second column, line 55, claim 21, for the word "electrodes" read co1ductorsg line 59, claim 25, for "conductors" read --electrodea--g and that the said Letters Patent should be read with this correctiontherein that the same may conform to the record of the case in the Patent Office. n

signed and sealed this 25th day of March, A. D. 19141.

Henry Van Arsdale, (Seal) Acting Commissioner of Petents. 

